How companies do it vs. what matters to investors
The process for actually calculating estimated income taxes can be quite complex and requires a team of accountants. The generally accepted accounting principles (GAAP) accounting rules that dictate how a company should report its financials to investors vary from the tax accounting rules required to calculate taxable income. Because of this -- and the overwhelming complexity of corporate accounting rules, in general -- the typical investor shouldn't worry about the methods and processes that accountants use to come up with the provision for income taxes.
Instead, investors should focus on how much the company is actually paying each year and how that amount compares with the company's competitors. Company management has a fiduciary responsibility to maximize shareholder value; in this case, that means paying the proper taxes due to the government without paying too much.